In oil field well logging operations, an armored electrical cable is connected between a surface sheeve or spool (usually on a logging truck) and a well or logging tool where the logging tool is passed through a well bore to obtain data relative to parameters of the well bore or the earth formations traversed by the well bore. The parameter data is recorded ultimately on a log or plot of the parameter or data as a function of depth. Depth is of course, a function of the length of the cable in the borehole to the well tool. In order to obtain the log correlation of parameter data relative to depth in a well bore it is necessary to measure the length of the cable in the borehole (and hence the position of a well tool) relative to the parameter data. Length of the cable can be measured mechanically with measuring wheels at the earth's surface and is commonly correlated with the length span of magnetic marks uniformly placed along the cable length. The magnetic marks are applied along the cable by placing the cable under a predetermined tension (typically 1000 pounds) and applying a magnetic mark at precise predetermined intervals (typically 1000 feet) along the entire length of cable. The magnetic mark typically has North and South poles spaced about 2 inches from one another with a magnetic strength of about 5 gauss.
In use of the cable, the cable is passed by a magnetic mark detector which detects the presence of the mark on the cable and produces a mark signal for use in the depth measurement recording. There are several problems involved with use of magnetic marks as a benchmark for length measurement of a cable. These problems include non-detection of magnetic marks on the cable because the mark has been removed or has become too weak for detection be the detector means. Existing detectors using coils or Hall devices give an indication of a magnetic mark only at relatively high gauss levels. No indication is given to the operator concerning the true gauss level of a mark thus the operator can begin an operation and find that the levels of detection used do not detect one or more marks. When this occurs, the length measurements can still be corrected provided that the operator or the equipment is aware of a missing mark. Where down hole positioning is involved, loss of a mark can seriously affect the operators ability to judge the location of the tool.
When a cable is new, it permanently stretches until it becomes "seasoned" which may be from 7 to 30 trips to a maximum tension value. The permanent stretch changes the spacing between the magnetic marks. For this reason and also when a number of marks have become non-detectable it is customary to remove the cable from service, erase the existing marks and apply new marks. In this process and on other instances, "false" or spurious marks can appear between the reference marks. Thus, an operator can have an error if a spurious mark is used in the length determination. This is particularly true where a false mark and non-detectable or missing mark are adjacent.
Another problem in mark detection is that conventional mark detectors are susceptible to the influence of the earth's magnetic field at a location where the logging operation is conducted and the earth's magnetic field can mask or adversely affect detection of both "strong" and "weak" marks, where "weak" marks are marks which are well below an original magnetic strength of a "strong" mark.